This invention relates to well logging and, more particularly, to an apparatus for well logging with electromagnetic energy.
The measurement of dielectric constant (or electric permittivity) of formations surrounding a borehole is known to provide very useful information about the formations. The dielectric constant of the different materials of earth formations vary widely (for example, 2.2 for oil, 7.5 limestone, and 80 for water), so measurement of dielectric properties is a useful means of formation evaluation. As an illustration, if the lithology and the water saturation of a particular formation are determined from conventional well logging techniques, or are otherwise known, it is recognized that the porosity should be determinable if the dielectric constant of the formation could be obtained. Similarly, if the lithology and porosity were given as knowns, information as to the degree of water saturation should be obtainable by measuring the dielectric constant of the formation.
A logging device which improved the art of measuring formation dielectric constant was the electromagnetic propagation tool as disclosed, for example, in the U.S. Pat. No. 3,944,910. As described in the referenced patent, that logging device includes a transmitter and spaced receivers mounted in a pad that is urged against the borehole wall. Microwave electromagnetic energy is transmitted into the formations, and energy which has propagated through the formations is received at the receiving antennas. The phase shift and attenuation of the energy propagating in the formations is determined from the receiver output signals. The dielectric constant and, if desired, the conductivity of the formations can then be obtained from the phase and attenuation measurements. Measurements are typically, although not necessarily, made on the formation invaded zone. The configuration of the antennas is an important aspect of successful operation of the described type of logging device. At the relatively high frequency of operation (for example 1100 MHz.) the signal attenuates quite rapidly, so it is important to have transmitting antennas which efficiently generate energy and inject it into the formations, and to have receiving antennas which efficiently receive energy which has propagated through the formations. Since the accuracy of the dielectric constant and conductivity measurements depends upon accurate measurements of attenuation and phase of the received signals, it is essential that the antennas operate in a stable manner over time and that the antennas are in, and remain in, a substantially balanced condition. Further, spurious signal components and the deleterious effects of fringing fields should be minimized.
In the abovereferenced U.S. Pat. No. 3,944,910 the antennas described in the electromagnetic propagation logging device are cavity-backed slot antennas which are filled with a dielectric material and include a probe which is an extension of the center conductor of the coaxial feed to a transmitting antenna (or from a receiving antenna, as the case may be) which extends across the cavity-backed slot and has a free end which terminates within a dielectric-filled recess in a side of the cavity-backed slot. Each antenna extends a cavity depth of a quarter wavelength into its metal housing. The length of the cavity-backed slot is a half wavelength.
The described type of cavity-backed slot antenna is inherently tuned at the operation frequency due to its cavity structure. At the relatively low signal levels involved, this tuning helps provide efficient operation at the selected frequency for obtainment of a reasonable signal-to-noise ratio. Some disadvantages are present, however, in the structure and operation of the described type of logging device. The dimensions of the cavity-backed slot are a sizeable fraction of a wavelength. As a consequence of the relatively large size, the phase-center of the slot is not well defined and, also, the slot opening can tend to interact substantially with electromagnetic waves propagating near the opening. The cavity-backed slot is inherently narrow band, and this limits the possibility of varying the operating frequency. The drilling borehole is a very difficult operating environment, and the logging device is subjected to a range of pressures, temperatures, and mechanical stresses. Applicant has noted that the incursion of fluid into the dielectric material can be a substantial cause of operating problems in the types of logging devices described above. The dielectric material exposed to the borehole environment can suffer the incursion of water. Water can enter through cracks in the dielectric material (the possibility of such cracks being increased by the hostile environment) and, even without cracks, the dielectric materials may have an inherent degree of porosity which is subject to the incursion of fluids. When a fluid (typically water) enters the dielectric material in the cavity-backed slot, the dielectric constant of the "wet" dielectric can increase substantially over its original "dry" value (since water has a much higher dielectric constant than the dielectric material utilized). Since the wavelength of the operating signal within the cavity-backed slot depends on the dielectric constant of the medium containing the wave, the wavelength of the energy will be changed (shortened in this case due to the high dielectric constant of water), and the operation of the cavity-backed slot antenna will degrade as the cavity goes out of resonance.
The incursion of fluid into the dielectric material of the antenna can also cause a problem because of its effect on the material around the free-ended probe in the described structure. In particular, the probe end in the recess of the antenna wall has a certain inherent capacitance, the value of which will be affected by the dielectric constant of the dielectric material. Again, if the dielectric material becomes "wet", the antenna characteristic will be changed.
It is among the objects of the present invention to generally improve operation of electromagnetic logging devices of the type utilized, inter alia, for determination of formation dielectric constant, and further to provide improved and more stable operation in a borehole environment.